Operator performance was primarily measured by assessing error rate, i.e. In practice, the position of vehicles within the camera field of view would be entirely random. The position of the target vehicle on the hard shoulder varied but was shown mainly at the far point of the field of view – the worst case scenario where the target is at its smallest. With a vehicle on the hard shoulder (stimulus clips). Without a vehicle on the hard shoulder and The video was captured using a digital camcorder, transferred to PC and edited into 20 s clips. Live CCTV feeds from appropriate CCTV cameras, with fields of view specified for ATM, were not available and hence representative video footage was captured during day and night (under motorway lighting) conditions. The ability to do back-of-the-envelope calculations is especially handy if an online calculator is inaccessible. However, it is instructive to at least be aware of the theory behind these calculations so that one appreciates that changing certain parameters can greatly affect system performance for a given scenario. Online calculators exist to help with CCTV system design (e.g., #), and these can take some of the pain out of the process. If the field-of-view is too wide, the resolution may be insufficient to meet a scenario-specific operational requirement (e.g., facial recognition). If the field-of-view is too narrow, there is a risk of missing a risk-relevant event. The operational implications of the field-of-view are obvious. (Reproduced with permission from .)įortunately one need not memorize camera focal lengths to select the appropriate CCTV camera, but it is helpful to understand that it is a combination of the lens focal length, the distance from the lens to the object, and the CCD sensor size that determines the field-of-view, a crucial CCTV feature with significant security implications. If the lens focal length is 3.5 mm and the camera uses a 1/3-in CCD, then the horizontal angle-of-view of the lens is given by 4.4 mm/3.5 mm = 1.25 rad or 72 degrees.įigure 11.6. We will later observe that the number of pixels-per-foot across the horizontal angle-of-view is a key specification for camera resolution. A 1/3-in CCD has a horizontal dimension of 4.4 mm. CCD w is often fixed, so the variable in determining the angle-of-view defaults to the lens focal length.įor example, a standard dimension for a CCTV camera CCD is 1/3 in. In other words, the angle-of-view is a function of the relevant dimension of the sensor (i.e., horizontal or vertical) divided by the lens focal length. It can be shown by geometric arguments that an approximate expression for the angle-of-view equals the CCD sensor dimension (CCD w), divided by the lens focal length ( f), or CCD w/ f. For example an angle of 1 rad is roughly equal to 180/3.14 ~57 degrees. To convert radians into degrees, multiply the number of radians (rad) by 180/π. As the reader is probably aware, there are 360 degrees in a circle. There are 2π radians in a complete circle, which can also be characterized in terms of degrees. The angle-of-view is typically expressed in radians. It is also important in determining the camera resolution because it relates to the density of pixels. The angle-of-view of a camera lens determines the angle of acceptance of the imaged scene along the relevant dimension of the CCD sensor (i.e., horizontal or vertical). The CCD is to a CCTV camera what the retina is to the human eye. The lens of the camera, the dimensions of the optical sensor (CCD), and the distance of the lens from the object determine the field-of-view. If full-area coverage of a lobby is required, a camera or series of cameras must be able to image the entire scene. Young, in The Science and Technology of Counterterrorism, 2015 11.6 Angle-of-View and Field-of-ViewĪ CCTV camera is not of much use if it is unable to image the required area as dictated by the operational requirements.
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